1. Field of the Invention
The present invention generally relates to printing equipment and processes. More particularly, this invention relates to a web finishing system suitable for use in inline and offline finishing systems in which multiple webs are simultaneously processed.
2. Description of the Related Art
In a typical inline printing and finishing apparatus used in the printing industry, a single paper web passes through a series of printing presses at high speeds before being printed (wetted), dried and cooled, after which the web undergoes a secondary finishing operation such as folding, perforating, gluing, die cutting and rotary cutting the printed web to a desired length. The speed at which a web can be printed and dried is typically higher than the speed at which finishing operations can be performed, such that the web finishing operation may artificially limit the speed at which finished printed material can be produced. As such, there are circumstances in which an offline web finishing system is desired.
If an offline web finishing system is used, printed webs are rewound at the end of the printing press line to form a preprinted roll ready for a secondary offline finishing operation. In the process of rewinding a preprinted web, it is difficult to maintain an even level of tension throughout the entire rewound web, even though the tension of the web is monitored through extreme care. Relatively small changes in web tension can cause significant print variation of the repeat length of a web's print image, which typically comprises multiple images that repeat as a set along the length of a printed web. Because paper is sensitive to environmental conditions such as humidity, another problem that arises with rewound webs is that during storage, the printed image may shrink, expand or do both within the same rewound web, depending on the conditions within the storage area. As a result, preprinted and rewound webs normally exhibit variations in the repeat length of its print image (“print image repeat length”) throughout its length.
For economy and/or other considerations, preprinted and rewound webs are unspooled and registered (aligned) with each other so that the webs can together undergo the finishing operations necessary to produce a finished product. For this purpose, preprinted webs have registration marks printed thereon that can be detected with a registration scanning system to synchronize the finishing equipment with the printed images on the web. Nonetheless, print image registration can be a significant problem in web finishing, especially when dealing with multiple webs. More particularly, it is difficult to register the print images of two or more preprinted and rewound webs because, for the reasons discussed above, each web typically has a different trend of print image length variations that can be localized or cumulative.
The conventional method of operating a web finishing system that involves multiple rewound webs is to print one web, referred to herein as the “primary” web, to achieve a generally precise print image repeat length, while other (“secondary”) webs are intentionally printed to have shorter image repeat lengths. This method requires stretching the images of the secondary web(s) in an attempt to match the primary web, or to shrink the image of the primary web to match the image of the secondary web through the finishing system. The former scenario is depicted in FIG. 1, which shows a primary web 10 and a secondary web 12 side-by-side instead of being registered with each other for purposes of clarity. As noted above, the primary and secondary webs 10 and 12 have print images 14 containing a series of individual images 20 and 22, respectively, that are repeated with each print image repeat length. As represented in FIG. 1, the secondary web 12 has been overstretched, such that the print image repeat length of the secondary web 12 is longer than that of the primary web 10, as evidenced by the phantom lines 16. In addition to such inaccuracies, the conventional method of stretching or shrinking webs may cause tension upset in the finishing operation and result in web break.
It is believed that a method does not exist in which an offline system is capable of dealing with the variation of the position of the print repeat and cumulative errors of the repeat length on webs that are aligned for simultaneously undergoing an operation. Accordingly, there is a need for a method capable of providing reliable and commercially acceptable finished products from multiple preprinted and rewound webs.